Diarylamine antioxidants are known and have been widely used to improve the thermal-oxidative stability and/or light induced degradation in numerous products used in engineering; for example, they can improve the performance properties in lubricants, hydraulic fluids, metal working fluids, fuels or polymers, just to name a few.
Commonly, these diarylamines have been alkylated, see for example, U.S. Pat. No. 2,943,112 which discloses an improved process for alkylating diphenylamine and U.S. Pat. No. 3,655,559 which discloses alkylated diphenylamines as stabilizers. Alkaryl substituted diphenylamines and phenylnapthylamines (such as α-methylstyryl-diphenylamine) are disclosed for example in U.S. Pat. Nos. 3,533,992; 3,452,056 and 3,660,290.
Synergist and antagonist combinations of antioxidants have been disclosed. Effective synergistic mixtures of antioxidants are typically compounds that intercept oxidation by two different mechanisms. For example, those in which one compounds functions as decomposer of peroxides and the other compound functions as an inhibitor of free radicals. Well known heterosynergism has been disclosed between sulfur and phosphorous containing compounds (such as sulfides, dithiocarbamates, phosphites and dithiophosphates) and aminic or phenolic antioxidants. U.S. Pat. No. 2,718,501 discloses a synergistic mixture of a sulfur-containing compound, such as a wax sulfide or dioctadecyl disulfide, and an aromatic amine compound having at least 2 aromatic rings, such as phenyl alpha-naphthyl amine, for use in preventing oxidation in lubricating oils. For example, U.S. Pat. No. 2,958,663 discloses an extreme pressure lubricant composition containing from 0.01 to 5 percent each of sulfurized oleic acid, C18-C22 alkenyl succinic acid, chlorinated paraffin wax containing from 20 to 60 percent chlorine, diphenylamine and N,N-salicylal-1,2-propylenediamine. U.S. Pat. No. 3,345,292 discloses stabilized alkyl substituted diaryl sulfides for use as functional fluids where the stabilizer can be diaryl amine or alkylated phenol. U.S. Pat. No. 4,032,462 discloses lubricants having improved antioxidancy having an oil soluble antimony compound and an oil soluble antioxidant selected from sterically hindered phenols and thiophenols, and aromatic amines, and mixtures of these antioxidants. U.S. Pat. No. 4,089,792 discloses lubricants having a an antioxidant mixture of a primary amine and an antioxidant selected from aromatic or alkyl sulfides and polysulfides, sulfurized olefins, sulfurized carboxylic acid esters and sulfurized ester-olefins. U.S. Pat. No. 4,102,796 discloses lubricants having an antioxidant mixture of aromatic and alkyl sulfides and polysulfides, sulfurized olefins, sulfurized carboxylic acid esters and sulfurized ester-olefins and a secondary aliphatic amine. U.S. Pat. No. 6,306,802 discloses of an antioxidant mixture containing a combination of an oil soluble molybdenum compound and an aromatic amine.
Commonly, antioxidant compounds which were intended to decompose hydroperoxides or peroxides (including sulfurized olefins, metal dithiocarbamates, dithiophospates, phosphites, thioesters, etc.) are increasingly difficult to incorporate into the finished lubricant due to the undesirable amounts of sulfur, phosphorous and ash content they add to the lubricating oil composition. It is known that sulfur, phosphorous and ash content may negatively impact pollution control devices (such as poisoning catalysts, etc.) which are finding increasing application. Thus, there is a need for new antioxidants and antioxidant systems which do not negatively impact pollution control equipment yet provide improved antioxidancy performance to handle the typical higher operating temperatures and longer service life desired. Synergistic antioxidant systems are particularly important since they reduce the overall additive impact. For example, some antioxidants such as diphenylamines cannot be used at relatively high concentrations since this may result in sedimentation or deposits in hot engine areas such as the diesel ring areas in diesel engines. Thus, one aspect of the invention is directed to an improved antioxidant system comprising a free radical antioxidant and an effective peroxide decomposer selected from an tetraalkyl-naphthalene-1,8-diamine.